Harsh operating conditions common to various systems can degrade and/or damage a surface of an article. An environmental barrier coating (EBC) is often deposited over the surface of the article to reduce or eliminate the degradation and/or damage. For example, one form of damage includes the degradation of a ceramic matrix composite (CMC) by water vapor in a gas stream. The water vapor reacts with silicon carbide to form silicon hydroxides. One common process of depositing the EBC is through thermal spraying, such as air plasma spraying.
During a conventional air plasma spraying, the EBC is deposited in an amorphous state. In the amorphous state, atoms of the EBC are not arranged in an ordered lattice. To increase performance of the coating, the amorphous structure can be crystallized, or formed into a crystalline structure, by a post-coating heat treatment of the coated article. The crystallization of the coating often produces a volume change in the coating, producing stresses that can lead to defects and/or delamination. The post-coating heat treatment of the article causes the EBC material to expand as the crystalline structure is formed. The expansion of the EBC material can cause various micro-structural defects such as micro-cracks, delamination of the EBC from the article, or a combination thereof. The delamination of the EBC introduces locations for EBC and/or article damage and/or failure.
One method of reducing or eliminating the defects formed during expansion of the EBC material includes extending the post-coating heat treatment to greater than 50 hours; however, this is time consuming and increases production costs. Other methods of avoiding the expansion of the EBC material include the use of an open box furnace to heat the article prior to, and concurrent with EBC deposition, and the use of electrical resistance heating to heat the article prior to, and concurrent with EBC deposition. The open box furnace is not suited to coating components with complex geometry or to a robust manufacturing process. Resistance heating forms non-uniform heating which produces local overheating and melting of regions of the article.
Coating processes and coated articles that do not suffer from one or more of the above drawbacks would be desirable in the art.